Patent application title: BATTERY

Abstract:

A battery comprising a first electrode, a second electrode, a separator
interposed between the first electrode and the second electrode, and an
electrolyte having lithium ion conductivity. The first electrode and the
second electrode are wound with the separator interposed therebetween to
form an electrode assembly. The first electrode includes a current
collector and an active material layer carried on one face of the current
collector. The active material layer includes columnar particles having a
bottom and a head, the bottom of the columnar particles being adhered to
the current collector. The head of the columnar particles is positioned
at an outer round side of the electrode assembly than the bottom.

Claims:

1. A battery comprising a first electrode, a second electrode, a separator
interposed between said first electrode and said second electrode, and an
electrolyte having lithium ion conductivity, said first electrode and
said second electrode being wound with said separator interposed
therebetween to form an electrode assembly, whereinsaid first electrode
includes a current collector and an active material layer carried on one
face of said current collector; andsaid active material layer includes
columnar particles having a bottom and a head, the bottom of said
columnar particles being adhered to said current collector, and the head
of said columnar particles being positioned at an outer round side of
said electrode assembly than the bottom.

2. The battery in accordance with claim 1, wherein said first electrode is
a negative electrode.

3. The battery in accordance with claim 1, wherein an angle formed between
a direction from the bottom toward the head of said columnar particles
and a direction (N) normal to said current collector is 20.degree. to
70.degree..

4. The battery in accordance with claim 1, wherein an angle formed between
a component parallel to said current collector of the direction from the
bottom toward the head of said columnar particles, and a winding axis of
said electrode assembly is 80.degree. or more and 100.degree. or less.

5. The battery in accordance with claim 1, wherein said columnar particles
are curved such that a current collector side thereof is projected.

6. The battery in accordance with claim 1, wherein said columnar particles
include at least one selected from the group consisting of a silicon
simple substance, a silicon alloy, a compound containing silicon and
oxygen, a compound containing silicon and nitrogen, a tin simple
substance, a tin alloy, a compound containing tin and oxygen, and a
compound containing tin and nitrogen.

7. A battery comprising a first electrode, a second electrode, a separator
interposed between said first electrode and said second electrode, and an
electrolyte having lithium ion conductivity, said first electrode and
said second electrode being wound with said separator interposed
therebetween to form an electrode assembly, whereinsaid first electrode
includes a current collector, a first active material layer carried on
one face of said current collector, and a second active material layer
carried on the other face of said current collector;said first active
material layer includes columnar particles A having a bottom and a head,
the bottom of said columnar particles A being adhered to said current
collector;said second active material layer includes columnar particles B
having a bottom and a head, the bottom of said columnar particles B being
adhered to said current collector;the head of said columnar particles A
is positioned at an outer round side of said electrode assembly than the
bottom; andthe head of said columnar particles B is positioned at the
outer round side of said electrode assembly than the bottom.

8. The battery in accordance with claim 7, wherein said first electrode is
a negative electrode.

9. The battery in accordance with claim 7, wherein an angle formed between
a component parallel to said current collector of a direction from the
bottom toward the head of said columnar particles A, and a component
parallel to said current collector of a direction from the bottom toward
the head of said columnar particles B is 0.degree. or more and 90.degree.
or less.

10. The battery in accordance with claim 7, wherein an angle formed
between the direction from the bottom toward the head of said columnar
particles A and a direction normal to said current collector is
20.degree. to 70.degree.; and an angle formed between the direction from
the bottom toward the head of said columnar particles B and the direction
normal to said current collector is 20.degree. to 70.degree..

11. The battery in accordance with claim 7, wherein an angle formed
between the component parallel to said current collector of the direction
from the bottom toward the head of said columnar particles A, and a
winding axis of said electrode assembly is 80.degree. or more and
100.degree. or less; and an angle formed between the component parallel
to said current collector of the direction from the bottom toward the
head of said columnar particles B, and the winding axis of said electrode
assembly is 80.degree. or more and 100.degree. or less.

12. The battery in accordance with claim 7, wherein said columnar
particles A are curved such that a current collector side thereof is
projected, and said columnar particles B are curved such that a current
collector side thereof is projected.

13. The battery in accordance with claim 7, wherein said columnar
particles A and said columnar particles B each include at least one
selected from the group consisting of a silicon simple substance, a
silicon alloy, a compound containing silicon and oxygen, a compound
containing silicon and nitrogen, a tin simple substance, a tin alloy, a
compound containing tin and oxygen, and a compound containing tin and
nitrogen.

Description:

RELATED APPLICATIONS

[0001]This application is the U.S. National Phase under 35 U.S.C. §
371 of International Application No. PCT/JP2006/320543, filed on Oct. 16,
2006, which in turn claims the benefit of Japanese Application No.
2005-306903, filed on Oct. 21, 2005, the disclosures of which
Applications are incorporated by reference herein.

TECHNICAL FIELD

[0002]The invention relates to batteries, and specifically relates to a
battery including an electrode including a current collector and an
active material layer carried on the current collector, in which the
active material layer includes columnar particles.

BACKGROUND ART

[0003]In recent years, as an electrode material for non-aqueous
electrolyte secondary batteries, a material containing a high capacity
element has been attracting interest. For example, a material containing
silicon (Si) or tin (Sn) has been attracting interest as a negative
electrode active material with high capacity. The theoretical discharge
capacity of Si is approximately 4199 mAh/g, which corresponds to an
amount approximately 11 times as large as the theoretical discharge
capacity of graphite.

[0004]However, these active materials, during the absorption of lithium
ions, undergo a great change in their structures and expand. As a result,
the active material particles break or the active material is peeled off
from the current collector. This results in a reduction in electron
conductivity between the active material and the current collector, which
may degrade battery characteristics (in particular, cycle
characteristics).

[0005]Under these circumstances, there has been proposed to use an oxide,
a nitride, an oxynitride, etc. containing Si or Sn. In active materials
including these, although the discharge capacity is slightly reduced, the
degrees of expansion and contraction are reduced. Further, there has been
proposed to provide an active material layer with a space for relieving
the expansion during the absorption of lithium ions (Patent Documents 1
to 3).

[0006]Patent document 1 proposes forming an active material layer
including columnar particles in a predetermined pattern on a current
collector. A photo resist method and plating techniques are employed in
forming a negative electrode active material layer. By forming an active
material in a columnar state, gaps are created in the active material
layer. This relieves the stress due to the expansion and the contraction
of the active material, and prevents the destruction of the active
material.

[0007]Patent Document 2 discloses an electrode including active material
particles slanting with respect to a direction normal to a current
collector. By slanting the active material particles with respect to a
direction normal to the current collector, the stress due to the
expansion and the contraction of the active material is relieved, and the
destruction and the peeling-off from the current collector of the active
material layer can be suppressed. This results in an improvement in
battery characteristics such as cycle characteristics.

[0008]Patent Document 3 discloses a method for growing active material
particles slanting with respect to a direction normal to a current
collector of continuous length. The current collector of continuous
length is transferred from a feeding roller to a film-forming roller. An
element (an active material source) capable of absorbing and desorbing
lithium is emitted from a target so as to be incident on the current
collector on the film-forming roller. Between the current collector and
the target, a mask for shielding the active material source is disposed
so that the active material source cannot be incident on the surface of
the current collector from a direction perpendicular thereto.

[0009]In the negative electrode as disclosed in Patent Document 1, the
active material particles (columnar particles) are allowed to stand
upright in a direction normal to the current collector. Therefore, during
the expansion of the active material, the electrode is subjected to
intensive pressure from the upper and lower directions. For example, from
an adjacent separator to the electrode, intensive pressure in the
direction normal to the current collector is applied. Moreover, since
individual columnar particles are separated by the gaps present among the
particles, the mechanical strength of the columnar particles is not
necessarily high. For this reason, the pressure from the upper and lower
directions deforms the micropores of the separator or destroys the active
material particles. As a result, the cycle characteristics or rate
characteristics of the battery are degraded.

[0010]The electrodes as disclosed in Patent Documents 2 and 3 are
effective in relieving the stress due to the expansion and the
contraction of an active material, and produce a certain level of effect
in improving battery characteristics. However, it is expected to improve
the battery characteristics through further stress relieving.

[0011]The invention intends to provide a highly reliable battery having
excellent characteristics by effectively relieving the stress due to the
expansion and the contraction of a high capacity active material.

Means for Solving the Problem

[0012]The invention relates to a battery including a first electrode, a
second electrode, a separator interposed between the first electrode and
the second electrode, and an electrolyte having lithium ion conductivity,
the first electrode and the second electrode being wound with the
separator interposed therebetween to form an electrode assembly, wherein
the first electrode includes a current collector and an active material
layer carried on one face of the current collector; and the active
material layer includes columnar particles having a bottom and a head,
the bottom of the columnar particles being adhered to the current
collector, and the head of the columnar particles being positioned at an
outer round side of the electrode assembly than the bottom.

[0013]In this battery, a preferred angle formed between a direction from
the bottom toward the head of the columnar particles (a growth direction
of the columnar particles) and a direction normal to the current
collector is 20° to 70°.

[0014]A preferred angle formed between a component parallel to the current
collector of the direction from the bottom toward the head of the
columnar particles and a winding axis of the electrode assembly is
80° or more and 100° or less.

[0015]It is preferable that the columnar particles are curved such that a
current collector side thereof is projected.

[0016]It is preferable that the columnar particles include at least one
selected from the group consisting of a silicon simple substance, a
silicon alloy, a compound containing silicon and oxygen, a compound
containing silicon and nitrogen, a tin simple substance, a tin alloy, a
compound containing tin and oxygen, and a compound containing tin and
nitrogen.

[0017]The invention also relates to a battery including a first electrode,
a second electrode, a separator interposed between the first electrode
and the second electrode, and an electrolyte having lithium ion
conductivity, the first electrode and the second electrode being wound
with the separator interposed therebetween to form an electrode assembly,
wherein the first electrode includes a current collector, a first active
material layer carried on one face of the current collector, and a second
active material layer carried on the other face of the current collector;
the first active material layer includes columnar particles A having a
bottom and a head, the bottom of the columnar particles A being adhered
to the current collector; the second active material layer includes
columnar particles B having a bottom and a head, the bottom of the
columnar particles B being adhered to the current collector; the head of
the columnar particles A is positioned at an outer round side of the
electrode assembly than the bottom; and the head of the columnar
particles B is positioned at the outer round side of the electrode
assembly than the bottom.

[0018]In this battery, a preferred angle formed between a component
parallel to the current collector of a direction from the bottom toward
the head of the columnar particles A (a growth direction of the columnar
particles A), and a component parallel to the current collector of a
direction from the bottom toward the head of the columnar particles B (a
growth direction of the columnar particles B) is 0° or more and
90° or less.

[0019]A preferred angle formed between the direction from the bottom
toward the head of the columnar particles A and a direction normal to the
current collector is 20° to 70°, and an preferred angle
formed between the direction from the bottom toward the head of the
columnar particles B and the direction normal to the current collector is
also 20° to 70°.

[0020]A preferred angle formed between the component parallel to the
current collector of the direction from the bottom toward the head of the
columnar particles A, and a winding axis of the electrode assembly is
80° or more and 100° or less; and a preferred angle formed
between the component parallel to the current collector of the direction
from the bottom toward the head of the columnar particles B, and the
winding axis of the electrode assembly is also 80° or more and or
less.

[0021]It is preferable that the columnar particles A are curved such that
a current collector side thereof is projected, and the columnar particles
B are also curved such that a current collector side thereof is
projected.

[0022]It is preferable that the columnar particles A and the columnar
particles B each include at least one selected from the group consisting
of a silicon simple substance, a silicon alloy, a compound containing
silicon and oxygen, a compound containing silicon and nitrogen, a tin
simple substance, a tin alloy, a compound containing tin and oxygen, and
a compound containing tin and nitrogen.

[0023]Although the invention is effective especially when the first
electrode is a negative electrode, the invention includes a case where
the first electrode is a positive electrode. Moreover, the invention
includes a case where the second electrode has a structure similar to
that of the first electrode as described above.

[0024]In the invention, a direction normal to the current collector means
a direction being perpendicular to the surface of the current collector
as well as departing from the surface of the current collector.
Microscopically, the surface of a current collector is rough in many
cases, but visually, it is flat. For this reason, a direction normal to
the current collector is uniquely determined.

[0025]In the invention, unless otherwise defined specifically, a direction
from the bottom toward the head of columnar particles is regarded as
identical with a growth direction of the columnar particles.

[0026]An angle β formed between a direction from the bottom toward
the head of columnar particles and a direction normal to the current
collector can be determined, for example, using an electron microscope
(SEM etc). In the case of using an electron microscope, an active
material layer is cut in parallel with a direction normal to the current
collector as well as in parallel with a growth direction of the columnar
particles, and a cross section thereof (hereinafter referred to as a
cross section C) is observed.

[0027]In the cross section C, mean lines corresponding to the surface of
the current collector and the surface of the active material layer are
determined. A straight line L which is at equal distance from the
determined two mean lines is obtained. The straight line L intersects
with a curve representing the contour of a columnar particle at two
points. At each of the two points, a tangent to the contour of the
columnar particle is determined. Angles β1 and β2 formed
between these tangents and the direction normal to the current collector
are determined. Then, the angle β formed between the direction from
the bottom toward the head of the columnar particles and the direction
normal to the current collector can be determined from
β=(β1+β2)/2. Here, a mean line is a term used in JIS
Standards (JIS B 0601-1994), which defines surface roughness Ra, meaning
a straight line determined from a mean value on a roughness chart.

[0028]Even in the case where the growth direction of columnar particles
fluctuates as going from the bottom toward the head, a component parallel
to the current collector of the growth direction of the columnar
particles is uniquely determined depending on production methods.
Accordingly, a cross section C is uniquely determined. For example, a
flat plane parallel to a vertical direction passing through a center of
an active material source to be evaporated and a point on the current
collector which is at the shortest distance from the center of the active
material source is determined. This flat plane intersects with the
current collector to form a straight line, which is in parallel with a
component parallel to the current collector of the growth direction of
the columnar particles.

[0029]Similarly, an angle γ formed between a component parallel to
the current collector of the direction from the bottom toward the head of
columnar particles and a winding axis of the electrode assembly is
uniquely determined. Similarly, an angle α formed between a
component parallel to the current collector of the direction from the
bottom toward the head of columnar particles A and a component parallel
to the current collector of the direction from the bottom toward the head
of columnar particles B is uniquely determined.

[0030]As for the angles β and γ, it is preferable to measure at
least 10 columnar particles to determine a mean value thereof. As for the
angle α also, it is preferable to measure at least 10 pairs of
columnar particles to determine a mean value thereof. It should be noted
that the angle β tends to become smaller gradually as the charge and
discharge of the battery proceeds. Therefore, as for the evaluation of
the angle β, it is preferable to use an electrode immediately after
production, an electrode included in an unused battery immediately after
production, or an electrode included in a battery having been subjected
to charge and discharge only 10 times or less.

[0031]In the case where the battery of the invention is a lithium
secondary battery, one of the first and the second electrodes is a
positive electrode capable of absorbing and desorbing lithium ions, and
the other is a negative electrode capable of absorbing and desorbing
lithium ions. The positive electrode and the negative electrode expand
during the absorption of lithium ions and contract during the desorption
of lithium ions. However, the expansion and the contraction of the
negative electrode are far greater than those of the positive electrode.
Therefore, in the invention, an excellent effect can be obtained in a
lithium secondary battery in which the negative electrode includes a
current collector and an active material layer carried on the current
collector, the active material layer includes columnar particles having a
bottom and a head, the bottom of the columnar particles is adhered to the
current collector, and the head of the columnar particles is positioned
at an outer round side of the electrode assembly than the bottom.

EFFECT OF THE INVENTION

[0032]The battery of the invention is capable of reducing the pressure to
be applied to the separator or the active material layer during the
expansion of the active material, and can effectively prevent the
occurrence of troubles in the battery. The effect of the invention
becomes remarkable particularly when a high capacity active material
whose expansion and contraction are evident is used. Reducing the
pressure to be applied to the separator or the active material layer
during the expansion of the active material makes it possible to maintain
the shape (suppress the deformation) of the active material particles
(columnar particles) as well as to secure the micropores in the
separator. This results in an improvement in rate characteristics and the
cycle characteristics of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 A set of schematic structural diagrams of an electrode
assembly included in a battery according to an embodiment of the
invention.

[0034]FIG. 2 A set of schematic structural diagrams of an electrode
assembly included in a battery according to another embodiment of the
invention.

[0035]FIG. 3 A diagram showing dynamic relations between a current
collector and columnar particles included in an active material layer.

[0036]FIG. 4 A perspective view conceptually showing one of the columnar
particles formed on a current collector of an electrode included an
electrode assembly.

[0037]FIG. 5 A diagram showing dynamic relations among a current
collector, columnar particles included in a first active material layer
carried on one face of the current collector, and columnar particles
included in a second active material layer carried on the other face of
the current collector.

[0038]FIG. 6A A schematic diagram showing relations among a current
collector, columnar particles included in a first active material layer
carried on one face of the current collector, and columnar particles
included in a second active material layer carried on the other face of
the current collector.

[0039]FIG. 6B A conceptual diagram showing an example of directions of
pressure applied to a separator during the expansion of columnar
particles, in a plane parallel to a current collector.

[0040]FIG. 6c A conceptual diagram showing another example of directions
of pressure applied to a separator during the expansion of columnar
particles, in a plane parallel to a current collector.

[0041]FIG. 7 A diagram showing examples of a columnar particle curved such
that a current collector side thereof is projected and a columnar
particle curved such that a current collector side thereof is depressed.

[0042]FIG. 8 A partially cross-sectional diagram of an example of an
electrode preferably used for a battery of the invention.

[0043]FIG. 9 A diagram conceptually showing only one of the columnar
particles included in the first active material layer and only one of the
columnar particles included in the second active material layer in FIG.
8.

[0044]FIG. 10 A conceptual diagram showing an example of the conventional
electrode.

[0045]FIG. 11 A cross-sectional schematic view showing an example of a
production apparatus for an electrode.

[0046]FIG. 12 A cross-sectional schematic view showing another example of
a production apparatus for an electrode.

[0047]FIG. 13 A cross-sectional schematic view showing still another
example of a production apparatus for an electrode.

[0048]FIG. 14 A cross-sectional schematic view showing yet another example
of a production apparatus for an electrode.

[0049]FIG. 15 A cross-sectional perspective view, partially developed, of
an example of a cylindrical battery.

[0050]FIG. 16 Sets of diagrams showing relations between an electrode
assembly and a slanted direction of columnar particles in a negative
electrode active material layer in Examples and Comparative Examples.

[0051]FIG. 17A A graph showing relations between the discharge capacity
and the number of charge/discharge cycles of batteries in Example 1 and
Comparative Example 1.

[0052]FIG. 17B A graph showing relations between the discharge capacity
and the number of charge/discharge cycles of batteries in Example 2 and
Comparative Example 2.

[0053]FIG. 18 A set of diagrams showing an embodiment of the invention in
the case where columnar particles have a zigzag shape.

[0054]FIG. 19 A set of diagrams showing an embodiment of the invention in
the case where columnar particles have a helical shape.

BEST MODE FOR CARRYING OUT THE INVENTION

[0055]Embodiments of the invention will be hereinafter described with
reference to the drawings.

[0056]FIG. 1 is a set of schematic structural diagrams of an electrode
assembly included in a battery according to an embodiment of the
invention.

[0057]FIG. 1(a) is a partially developed diagram viewed from the bottom of
one side of a cylindrical electrode assembly 11. As is shown in FIG.
1(a), the electrode assembly 11 includes a band-shaped first electrode
12, a band-shaped second electrode 13, and a band-shaped separator 14
disposed between these electrodes. The first electrode 12 and the second
electrode 13 are wound with the separator 14 interposed therebetween. It
is preferable that the width of the band-shaped separator 14 is larger
than those of the band-shaped first electrode 12 and the band-shaped
second electrode 13, in light of securing insulation between the first
electrode and the second electrode.

[0058]FIG. 1(b) is a magnified schematic diagram of an area encircled by
the dashed line X in FIG. 1(a), showing a cross-section of the first
electrode 12. The cross-section of the second electrode 13 may be
configured similarly to or differently from the first electrode. The
first electrode 12 includes a current collector 15 and an active material
layer 16 carried on one face of the current collector. The active
material layer 16 includes columnar particles 18 having a bottom 18a and
a head 18b, and the bottom 18a of the columnar particles 18 is adhered to
the current collector 15. The head 18b of the columnar particles 18 is
positioned at the outer round side (Do) of the electrode assembly 11 than
the bottom 18a.

[0059]An axis 18c from the bottom 18a toward the head 18b of the columnar
particles 18 is slanted with respect to a direction N normal to the
current collector 15. Further, a point P on the axis 18c, as going from
the bottom 18a toward the head 18b, moves from the inner round side (Di)
to the outer round side (Do) of the electrode assembly 11.

[0060]FIG. 2 is a set of schematic structural diagrams of an electrode
assembly included in a battery according to another embodiment of the
invention.

[0061]FIG. 2(a) is a partially developed diagram viewed from the bottom of
one side of a cylindrical electrode assembly 21. As is shown in FIG.
2(a), the electrode assembly 21 includes a band-shaped first electrode
22, a band-shaped second electrode 23, and a band-shaped separator 24
disposed between these electrodes. The first electrode 22 and the second
electrode 23 are wound with the separator 24 interposed therebetween. It
is preferable that the width of the band-shaped separator 24 is larger
than those of the band-shaped first electrode 22 and the band-shaped
second electrode 23, in light of securing insulation between the first
electrode and the second electrode.

[0062]FIG. 2(b) is a magnified schematic diagram of an area encircled by
the dashed line Y in FIG. 2(a), showing a cross-section of the first
electrode 22. The cross-section of the second electrode 23 may be
configured similarly to or differently from the first electrode. The
first electrode 22 has a current collector 25 and a first active material
layer 26 carried on one face of the current collector and a second active
material layer 27 carried on the other face. The first active material
layer 26 includes columnar particles A 28 having a bottom 28a and a head
28b, the bottom 28a of the columnar particles 28 being adhered to the
current collector 25. Similarly, the second active material layer 27
includes columnar particles B 28' having a bottom 28a' and a head 28b',
the bottom 28a' of the columnar particles 28' being adhered to the
current collector 25. The head 28b of the columnar particles 28 is
positioned at the outer round side (Do) of the electrode assembly 21 than
the bottom 28a. The head 28b' of the columnar particles 28' is positioned
at the outer round side (Do) of the electrode assembly 21 than the bottom
28a'.

[0063]An axis 28c from the bottom 28a toward the head 28b of the columnar
particles 28 is slanted with respect to a direction N normal to the
current collector 25. Further, a point Q on the axis 28c, as going from
the bottom 28a toward the head 28b, moves from the inner round side (Di)
to the outer round side (Do) of the electrode assembly 21.

[0064]Similarly, an axis 28c' from the bottom 28a' toward the head 28b' of
the columnar particles 28' is slanted with respect to a direction N'
normal to the current collector 25. Further, a point Q' on the axis 28c',
as going from the bottom 28a' toward the head 28b', moves from the inner
round side (Di) to the outer round side (Do) of the electrode assembly
21.

[0065]It is not necessary that the columnar particles be particles of a
strict cylindrical or prismatic shape, but a roughly columnar shape will
suffice. Further, the diameter (width) of the columnar particles may be
varied in the lengthwise direction thereof. The diameter of the columnar
particles may be increased as distanced from the connected portion with
the current collector (the bottom). The columnar particles may be curved.

[0066]Here, the inner round side of an electrode assembly is a position on
the electrode that is closer to a winding axis of the electrode assembly.
The winding axis of the electrode assembly is a position from which the
winding of a first electrode and a second electrode with a separator
interposed therebetween is started, and corresponds to the center of the
electrode assembly. The outer round side of an electrode assembly is a
position on the electrode that is farther from the winding axis of the
electrode assembly (closer to a position at which the winding is
completed).

[0067]An axis from the bottom toward the head of a columnar particle is
synonymous with a center line of the columnar particle in its
cross-section. When the columnar particle is of a cylindrical shape, the
axis from the bottom toward the head corresponds to the center axis of
the cylinder.

[0068]The effect of the invention will be described with reference to FIG.
3. FIG. 3 shows dynamic relations between a current collector 35 and
columnar particles 38 included in an active material layer. In this
diagram, only one columnar particle 38 is conceptually shown for
convenience. A head 38b of the columnar particle 38 is positioned at the
outer round side of the electrode assembly than a bottom 38a. Here, the
outer round side (Do) of the electrode assembly is the left side of FIG.
3, and the inner round side (Di) is the right side. A direction from the
bottom 38a toward the head 33b of the columnar particle 38 and a
direction N normal to a current collector 35 form an angle β3.

[0069]When the columnar particle 38 expands, for example, by absorbing
lithium ions, a force (F3') in an oblique direction acts on a point R of
force application in the bottom 38a. However, in the electrode assembly,
the head 38b of the columnar particle 38 is pressed with the separator,
etc. Therefore, with respect to the current collector 35, a force (F3) in
an opposite direction to F3' acts on the point R of force application. At
this time, a component f3 parallel to the current collector 35 of F3 acts
in such a manner as to push the current collector 35 toward the inner
round side (Di) of the electrode assembly. As a result, in FIG. 3, the
current collector moves toward the inner round side of the electrode
assembly as a whole.

[0070]When the electrode moves toward the inner round side of the
electrode assembly as a whole, looseness occurs in the electrode
assembly. Specifically, in the invention, when the active material
expands, the electrode assembly slightly gets loose, which creates minor
gaps in the electrode assembly. This relieves the stress caused by the
expansion of the active material, and thus suppresses the breakage in the
active material layer. Moreover, the pressure from the electrode to the
separator is weakened, which makes it easy for the separator to maintain
the shape of its micropores.

[0071]The angle β formed between an axis from the bottom toward the
head of columnar particles and a direction normal to the current
collector (angle β3 in FIG. 3) is preferably 20° or more and
70° or less, and more preferably 25° or more and 50°
or less. The each angle β, in all the columnar particles included in
the active material layer, may be the same or different. However, it is
preferable that the angle β of every columnar particle falls within
the range of 20° or more and 70° or less. When the angle
β is less than 20°, the direction of a force produced during
the expansion of the active materiel (F3' in FIG. 3) approaches the
direction normal to the current collector, and thus the amount of
movement of the current collector caused by the expansion of the active
material is reduced, for example, to approximately one-third or less.
This reduces the effect of the invention. On the other hand, when the
angle β exceeds 70°, the adhering strength between the
current collector and the bottom of the columnar particles is reduced,
and thus the effect of the invention is reduced.

[0072]In addition, in the case of increasing the angle β to greater
than 70°, there is difficulty in forming an active material layer
by vapor phase process. For example, the incident direction to the
current collector of the vapor of an active material source must be made
closer to the direction substantially parallel to the surface of the
current collector (for example, approximately within 10°). As a
result, the utilization efficiency of the active material source is
reduced. As such, this is disadvantageous in terms of actual
productivity.

[0073]FIG. 4 is a perspective view conceptually showing only one of the
columnar particle 48 formed on a current collector of an electrode 42
included an electrode assembly 41. FIG. 4 shows a relation between a
component d4 parallel to the current collector of a direction D4 from the
bottom toward the head of the columnar particle 48 (a growth direction of
the columnar particle 48), and a winding axis A4 of the electrode
assembly. A straight line L passing through a point S in the bottom of
the columnar particle 48 and being in parallel with the winding axis A4
forms an angle γ4 with the component d4 parallel to the current
collector of the growth direction D4 of the columnar particle 48. The
angle γ4 is synonymous with the angle γ formed between a
component parallel to the current collector of the growth direction D4
and the winding axis A4 of the electrode assembly 41. Here, in the point
S, the inner round side is the direction shown in an arrow of dotted line
(the left side of FIG. 4) and an outer round side is the direction
opposite thereto. Accordingly, the direction of d4 coincides with a
direction from the inner round side toward the outer round side in the
electrode assembly.

[0074]The angle γ formed between a component parallel to the current
collector of a direction from the bottom toward the head of columnar
particles (a growth direction of columnar particles) and a winding axis
of the electrode assembly is preferably around 90°, for example,
80° or more and 100° or less. In other words, it is
preferable that the component parallel to the current collector of the
direction from the bottom toward the head of columnar particles is
perpendicular or nearly perpendicular to the winding axis. The angle
γ of around 90° allows easy movement of the current
collector during the expansion of the active material. As a result,
looseness easily occurs in the electrode assembly, and this makes it easy
to relieve the expansion stress of the active material and to secure the
micropores in the separator. As such, an effective improvement in cycle
characteristics and rate characteristics of the battery can be expected.

[0075]In the case where the active material layer is formed on both faces
the current collector, the effect of the invention is increased. FIG. 5
shows dynamic relations among a current collector 55, columnar particles
58 included in a first active material layer carried on one face of the
current collector, and columnar particles 58' included in a second active
material layer carried on the other face. In this diagram, only one
columnar particle 58 included in the first active material layer and only
one columnar particle 58' included in the second active material layer
are conceptually illustrated for convenience. The inner round side (Di)
of the electrode assembly is the right side of FIG. 5 and the outer round
side (Do) is the left side.

[0076]In the columnar particle 58 and the columnar particle 58' on both
faces of the current collector 55, forces (f5 and f5') to move the
current collector 55 toward the inner round side of the electrode
assembly are produced during the expansion for the same reason as
described in the case of FIG. 3. This means that the force to move the
electrode toward the inner round side is doubled as compared with the
case where the active material layer includes columnar particles on only
one face. Consequently, the effect of the invention is increased.

[0077]FIG. 6A is a schematic diagram showing relations among a current
collector 65, columnar particles 68 included in a first active material
layer carried on one face of the current collector, and columnar
particles 68' included in a second active material layer carried on the
other face. A direction D6 from a bottom 68a toward a head 68b of the
columnar particle 68 and a direction N normal to the current collector 65
form an angle β6. Similarly, a direction D6' from a bottom 68a'
toward a head 68b' of the columnar particle 68' and a direction N' normal
to the current collector 65 form an angle β6'.

[0078]An angle α formed between a component d6 parallel to the
current collector 65 of D6 and a component d6' parallel to the current
collector 65 of D6' is preferably 0° or more and 90° or
less. When the columnar particles expand, pressure is applied to the
separators adjacent to both faces of the electrode. The components
parallel to the current collector of the pressure applied to the
separators act in the directions of d6 and d6'. When the angle α is
90°, a force applied to the separator adjacent to one face of the
electrode and a force applied to the separator adjacent to the other face
of the electrode are orthogonal to each other (See FIG. 6B). On the other
hand, when the angle α is 0°, forces parallel to each other
are applied to the separator on both sides (See FIG. 6c). As a reaction
to this, also to the current collector 65, an orthogonal force is applied
when the angle α is 90°, and a parallel force is applied
when the angle α is 0°. The forces applied to the current
collector at this time act in the opposite directions to d6 and d6'. FIG.
6B and FIG. 6c each show a relation between d6 and d6' in a plane
parallel to the current collector.

[0079]In view of suppressing the occurrence of wrinkles on the separator
and the current collector during the expansion of the columnar particles,
the angle α is preferably 0° or more and 60° or less,
more preferably 0° or more and 30° or less, and most
preferably 0°.

[0080]It is preferable that the structures of the active material layers
on both faces of the current collector are substantially symmetric to
each other. For example, in the case of FIG. 6A, it is preferable that
the angle α=0° and the angle β6=β6' are satisfied,
the thickness of the first active material layer carried on one face of
the current collector 65 and the thickness of the second active material
layer carried on the other face are substantially equal. In such a
symmetric state, it is not necessary that the individual columnar
particles are perfectly plane-symmetric about the current collector, but
it will suffice if the active material layers on both faces, as a whole,
are plane-symmetric in average.

[0081]The columnar particles may be curved. In other words, the columnar
particles may be formed in a bow shape. For example, the columnar
particles may be curved such that the current collector side thereof is
projected or the current collector thereof is depressed. Among these, it
is preferable that the columnar particles are curved such that the
current collector side thereof is projected.

[0082]FIG. 7 shows examples of a columnar particle 78A curved such that
the current collector 75 side thereof is projected and a columnar
particle 78B curved such that the current collector 75 side thereof is
depressed. In the case where columnar particles are curved such that the
current collector side thereof is projected, compared with the case where
the current collector side thereof is depressed, during the expansion of
the columnar particles, the electrode is moved easily (the electrode
assembly gets easily loose). This is because that a component (f7A)
parallel to the current collector of a force (F7A) produced in the
vicinity of the bottom of the columnar particle 78A is greater than a
component (f7A') parallel to the current collector of a force (F7A')
produced in the vicinity of the head of the columnar particle, and this
difference makes the electrode move easily. In the case where the
columnar particles are curved such that the current collector side
thereof is depressed, conversely, a component (f7B) parallel to the
current collector of a force (F7B) produced in the vicinity of the bottom
of the columnar particle is smaller than a component (f7B') parallel to
the current collector of a force (F7B') produced in the vicinity of the
head.

[0083]FIG. 8 is a partially cross-sectional diagram of an example of an
electrode preferably used for a battery of the invention. An electrode 80
has a first active material layer 81 formed on a first face (the upper
face in FIG. 8) of a sheet-like current collector 82 and a second active
material layer 81' formed on the other face (the lower face in FIG. 8) of
the current collector 82. The first active material layer 81 includes a
plurality of particles 84 slanting with respect to a direction N normal
to the current collector 82. Similarly, the second active material layer
81' includes a plurality of particles 84' slanting with respect to a
direction N' normal to the current collector 82. The plurality of
particles 84 and the plurality of particles 84' are both grown in a bow
shape such that the current collector side thereof is projected.

[0084]FIG. 9 conceptually shows only one of columnar particles 84 included
in the first active material layer and only one of columnar particle 84'
included in the second active material layer. A growth direction D9 of
the particle (i.e., a direction from the bottom toward the head of the
particle 84) forms an angle β9 with a normal direction N. Similarly,
a growth direction D9' of the particle 84' (i.e., a direction from the
bottom toward the head of the particle 84') forms an angle β9' with
a normal direction N'. Here, a component parallel to the current
collector 82 of D9 is denoted by d9. Similarly, a component parallel to
the current collector 82 of the direction D9' is denoted by d9'. An angle
α formed between the direction d9 and the direction d9' is
0° or more and 90° or less. The angle α is preferably
0° or more and 60° or less, more preferably 0° or
more and 30° or less, and most preferably 0°.

[0085]It is not necessary that the angles β9 and β9' are the
same. The angles β9 and β9' each are preferably 20° or
more and 70° or less, and more preferably 25° or more and
50° or less. It is also not necessary that every particle 84 in
the first active material layer 81 has the same angle β9, but it
will suffice if each particle has an angle of 20° or more and
70° or less. Similarly, it is not necessary that every particle
84' in the second active material layer 81' has the same angle β9',
but it will suffice if each particle has an angle of 20° or more
and 70° or less.

[0086]In the case of an electrode 100 as shown in FIG. 10, a first active
material layer 101 and a second active material layer 101' are carried on
a current collector 102, and particles 104 and 104' included in these
active material layers are formed in parallel with directions N and N'
normal to the current collector 102. In a battery using such the
electrode 100, during the expansion of the active material, pressure is
applied to the separator and the electrode in a direction perpendicular
thereto. On the other hand, in a battery using such the electrode 80 as
shown in FIG. 8, pressure is applied to the separator and the electrode
in a direction oblique thereto. In the latter case, the separator and the
active material layer will suffer less damage. As a result, a battery
excellent in rate characteristics and cycle characteristics can be
obtained.

[0087]The active material included in the active material layer is not
particularly limited as long as it electrochemically reacts with lithium.
However, in the case of the negative electrode active material, it is
preferable that the material includes at least one selected from the
group consisting of a silicon simple substance, a silicon alloy, a
compound containing silicon and oxygen, a compound containing silicon and
nitrogen, a tin simple substance, a tin alloy, a compound containing tin
and oxygen, and a compound containing tin and nitrogen, since the
material as such has a comparatively high reactivity with lithium and is
expected to have a high capacity. When these active materials are used,
the effect of the invention becomes remarkable.

[0088]In the case of the positive electrode active material, it is
preferable that the material includes, for example, a transition metal
oxide. For example, a lithium-containing transition metal oxide such as
lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2) or
lithium manganate (LiMn2O4) may be used, but not limited
thereto. In the case where the negative electrode active material layer
includes columnar particles slanting with respect to a direction normal
to the current collector, the positive electrode active material layer
may be composed of columnar particles as in the case of the negative
electrode active material layer, or composed of a material mixture
including a positive electrode active material and a binder.

[0089]The thickness of the active material layer, although being dependent
on the performance of a battery to be fabricated, is in the range
approximately 3 to 40 μm. If the thickness of the active material
layer is less than 3 μm, the proportion of the active material in the
entire battery becomes small, and the energy density of the battery is
reduced. On the other hand, if the thickness of the active material layer
exceeds 40 μm, the stress in the interface between the current
collector and the active material layer is increased, causing a
possibility of deformation of the current collector, and the like.

[0090]In light of the reactivity between the active material and lithium,
it is preferable that the active material is amorphous or of low
crystallinity. The term "low crystallinity" as used here refers to a
state in which the particle size of crystal grains (crystallites) is 50
nm or less. The particle size of crystal grains is calculated by
Scherrer's formula using half width of the highest peak of intensity in a
diffraction pattern obtained by X-ray diffraction analysis. The term
"amorphous" is used when no sharp peak is observed in the range of
2θ=15 to 40° in a diffraction pattern obtained by X-ray
diffraction analysis, but a broad peak (for example, a halo pattern) is
observed.

[0091]For the current collector of the negative electrode, for example, a
metal foil containing cupper, nickel and the like may be used. For the
current collector of the positive electrode, for example, a metal foil
containing aluminum, nickel, titanium and the like may be used. It is
preferable that the metal foil is a sheet of continuous length. In light
of the strength of the current collector, the volume efficiency of the
battery, the ease of handling of the current collector, and the like, it
is preferable that the thickness of the metal foil is 4 to 30 μm, and
more preferably 5 to 10 μm. Although the surface of the metal foil may
be smooth, a metal foil with a rough surface having a surface roughness
Ra of approximately 0.1 to 4 μm may be used in order to increase the
adhesion strength with the active material layer. The rough surface of
the metal foil also serves to form gaps between the columnar particles
included in the active material layer. In view of the adhesion strength
with the active material layer, costs, and the like, it is preferable to
use a metal foil of Ra=0.4 to 2.5 μm.

[0092]Examples of a method for fabricating an electrode for use in the
invention will be hereinafter described.

[0093]The electrodes as shown in FIGS. 1 to 9 are obtained by allowing an
active material layer to be carried on a current collector in a
predetermined method. The method for allowing an active material layer to
be carried on a current collector is not particularly limited as long as
the method can form columnar particles slanting with respect to a
direction normal to the current collector. However, it is preferable to
use a dry process such as a vapor deposition method, a sputtering method
or a CVD method. For example, by evaporating an active material source so
that the vapor flux is incident obliquely on the surface of the current
collector, an active material layer including columnar particles slanting
in a direction normal to the current collector can be obtained.

[0094]FIG. 11 is a cross-sectional schematic view showing an example of a
production apparatus for an electrode. A production apparatus 110
comprises a vacuum chamber 111 and an exhaust pump (not shown) for
keeping the interior thereof under vacuum. A flat fixing table 114 is
placed above a container 113 containing an active material source 112 in
such a manner that the table forms an angle θ with a horizontal
plane. On the surface of the fixing table 114, a current collector 115 is
fixed. A heating means is used to heat and evaporate the active material
source.

[0095]In the case of forming an active material layer containing an oxide
or a nitride, an active material source of the oxide or the nitride may
be evaporated directly, or an active material source not containing
oxygen or nitrogen (for example, silicon or tin) may be evaporated in an
oxygen atmosphere or a nitrogen atmosphere. In the case of forming an
active material layer on both faces of the current collector, after a
first active material layer is formed on one face of the current
collector, the current collector is turned upside down to form a second
active material layer on the other face.

[0096]FIG. 12 is a cross-sectional schematic view showing another example
of a production apparatus for an electrode. A production apparatus 120 is
suitable for the case of continuously forming an active material layer on
a current collector (metal foil) of continuous length. The production
apparatus 120 comprises a vacuum chamber 121 and an exhaust pump 122 for
keeping the interior thereof under vacuum. A current collector 124 of
continuous length send from a feeding roller 123 is transferred onto
transfer rollers 125a and 125b and runs along the periphery of a
cylindrical can roller 126. The current collector 124 and the can roller
126 are shielded from below with a shielding plate 128 having an opening.
The opening of the shielding plate 128 is provided in such a manner that
it is positioned between the transfer roller 125b and the can roller 126.
In this state, a container 129 containing an active material source 129a
is placed below the opening of the shielding plate 128 and the active
material source is evaporated. By doing this, during the time when the
current collector 124 moves obliquely from the transfer roller 125b until
it reaches the periphery of the can roller 126, the vapor of the active
material source supplied from below is incident obliquely on the surface
of the current collector. Thereafter, the current collector carrying an
active material layer (electrode) is transferred to transfer rollers 125c
and 125d, and then wound on a winding roller 127.

[0097]An incident angle to the surface of the current collector of the
vapor of the active material source supplied from below (an angle formed
between a direction normal to the current collector and an incident
direction of the vapor of the active material source) is gradually
reduced as the current collector from the transfer roller 125b approaches
the periphery of the can roller 126. Consequently, the columnar particles
are curved such that the current collector side thereof is projected. If
the transfer direction of the current collector is reversed, the columnar
particles are curved such that the current collector side thereof is
depressed. Moreover, as the current collector from the transfer roller
125b approaches the periphery of the can roller 126, the vapor amount of
the active material source present in the vicinity of the current
collector is increased. It should be noted that, in association with the
growth of the columnar particles, since the exposure of the head of the
columnar particles is increased, the diameter of the columnar particles
around the head becomes greater than that around the bottom.

[0098]FIG. 13 is a cross-sectional schematic view showing still another
example of a production apparatus for an electrode. A production
apparatus 130 comprises a vacuum chamber 131 and an exhaust pump 132 for
keeping the interior thereof under vacuum. From a gas-introducing pipe
1302, oxygen or nitrogen can be introduced in the interior of the vacuum
chamber 131 as needed. A current collector 134 of continuous length send
from a feeding roller 133 passes through a transfer roller 135a and runs
along the periphery of a cylindrical first can roller 136. Thereafter,
the current collector 134 passes through transfer rollers 135b to 135e
and runs on the periphery of a cylindrical second can roller 137 in such
a state that the current collector is turned upside down. Finally, the
current collector passes through a transfer roller 135f and is wound on a
winding roller 138.

[0099]The first can roller 136 and the second can roller 137 are shielded
from below with a shielding plate 139 having an opening. The opening of
the shielding plate 139 is provided in such a manner that it is
positioned between the periphery of the first can roller 136 and the
periphery of the second can roller 137. In this state, a container 1301
containing an active material source 1301a is placed below the opening of
the shielding plate 139 and the active material source is evaporated. The
active material source is heated by a heater (not shown) and evaporated.

[0100]The evaporated active material source passes through the opening of
the shielding plate 139 and is incident on the peripheries of the first
can roller 136 and the second roller 137. At this time, the active
material source is incident from a direction slanting with respect to a
direction normal to the current collector 134. On the periphery of the
first can roller 136, an active material is deposited on one face of the
current collector; and on the periphery of the second roller 137, the
active material is deposited on the other face of the current collector.

[0101]An incident angle to the surface of the current collector of the
vapor of the active material source supplied from below (an angle formed
between a direction normal to the current collector and an incident
direction of the vapor of the active material source) is gradually
reduced as the current collector moves downward along the periphery of
the first can roller 136 or the second can roller 137. Consequently, the
columnar particles are curved such that the current collector side
thereof is projected. It should be noted that in the case where an active
material is deposited on the current collector moving along the periphery
of the can roller, compared with the case where an active material is
deposited on the current collector moving in a straight line as in the
production apparatus of FIG. 12, the degree of curve of the columnar
particles can be increased and the utilization efficiency of the vapor of
the active material source is enhanced. Moreover, as the current
collector moves downward along the periphery of the first can roller 136
or the second can roller 137, the vapor amount of the active material
source present in the vicinity of the current collector is increased. It
should be noted that, in association with the growth of the columnar
particles, since the exposure of the head of the columnar particles is
increased, the diameter of the columnar particles around the head becomes
greater than that around the bottom.

[0102]FIG. 14 is a cross-sectional schematic view showing yet another
example of a production apparatus for an electrode. A production
apparatus 140 comprises a vacuum chamber 141 and an exhaust pump 142 for
keeping the interior thereof under vacuum. From a gas-introducing pipe
1402, oxygen or nitrogen can be introduced in the interior of the vacuum
chamber 141 as needed. A current collector 144 of continuous length send
from a feeding roller 143 passes through transfer rollers 145a and 145b
and runs along the periphery of a cylindrical first can roller 146.
Thereafter, the current collector 144 passes through transfer rollers
145c to 145h and runs on the periphery of a cylindrical second can roller
147 in such a state that the current collector is turned upside down.
Finally, the current collector passes through transfer rollers 145i and
145j and is wound on a winding roller 148.

[0103]The first can roller 146 and the second can roller 147 are shielded
from below with a shielding plate 149 having an opening. The opening of
the shielding plate 149 is provided in such a manner that it is
positioned between the periphery of the first can roller 146 and the
periphery of the second can roller 147. In this state, a container 1401
containing an active material source 1401a is placed below the opening of
the shielding plate 149 and the active material source is evaporated.

[0104]In the production apparatuses of FIG. 13 and FIG. 14, the diameters
of the first can roller and the second can roller are equal, and the
positions of the first can roller and the second can roller are symmetric
to each other with respect to the position of the active material source.
Accordingly, the growth directions of the columnar particles in the first
active material layer formed on one face of the current collector and the
columnar particles in the second active material layer formed on the
other face are substantially symmetric to each other. If the first can
roller and the second can roller are placed asymmetrically to each other
with respect to the active material source, the growth directions of the
columnar particles in the first active material layer and the columnar
particles in the second active material layer will be asymmetric to each
other.

[0105]The active material source is heated with a heater (not shown) such
as a resistance heater, an induction heater or an electron beam heater.
Such heating allows silicon or tin to evaporate. In the case of forming
an active material layer on both faces of the current collector, after a
first active material layer is formed on one face of the current
collector, a second active material layer is formed on the other face.
For a container for containing an active material source, a crucible or
the like may be used.

[0106]In the case where oxygen gas or hydrogen gas is introduced into the
vacuum chamber to evaporate silicon or tin in an oxygen atmosphere or a
nitrogen atmosphere, an active material layer including a compound
containing silicon and oxygen, a compound containing silicon and
nitrogen, a compound containing tin and oxygen, a compound containing tin
and nitrogen, and the like can be formed.

[0107]Although the foregoing methods for producing an electrode are
suitable particularly for the case of fabricating a negative electrode, a
similar method to these, with modification as needed, may be used also in
the case of fabricating a positive electrode.

[0108]The electrodes obtained by the foregoing production methods are
usually formed in a wound state, that is, have a roll shape. At this
time, the head of the columnar particles is positioned at the outer round
side or the inner round side of the electrode roll than the bottom.
Thereafter, lithium is vapor-deposited on the active material layer as
needed. This operation is typically preformed in order to compensate for
the irreversible capacity amount of the active material.

[0109]The vapor deposition of lithium can be performed using metallic
lithium in place of the active material source in the same manner as in
the operation of vapor-depositing an active material on the current
collector. Accordingly, the electrode after the vapor deposition of
lithium is also formed in a roll shape, and the head of the columnar
particles is positioned at the outer round side or the inner round side
of the electrode roll than the bottom.

[0110]Thereafter, an operation of cutting the electrode in a predetermined
width is typically performed. This operation includes steps of feeding an
electrode roll, cutting and winding it. Accordingly, the electrode after
cutting is also formed in a roll shape, and the head of the columnar
particles is positioned at the outer round side or the inner round side
of the electrode roll than the bottom. It is preferable that immediately
before forming an electrode assembly, the head of the columnar particles
is positioned at the inner round side of the electrode roll than the
bottom. By starting the winding of a positive electrode, a negative
electrode and a separator in this state, the bottom of the columnar
particles comes closer to the winding axis of the electrode assembly than
the head. Accordingly, an electrode assembly in which the head of the
columnar particles is positioned at the outer round side of the electrode
assembly than the bottom can be easily obtained.

[0111]For forming an electrode assembly, a positive electrode roll, a
negative electrode roll and two separator rolls are usually used. A
separator fed from one of the two separator rolls is interposed between
the positive electrode and the negative electrode, and a separator fed
from the other one of the two separator rolls is disposed outside the
positive electrode or the negative electrode, namely, four layers in
total are wound at the same time. In this step, the electrode including
columnar particles slanting with respect to a direction normal to the
current collector is wound such that the bottom of the columnar particles
comes closer to the winding axis. As a result, the head of the columnar
particles included in the electrode assembly is positioned at the outer
round side of the electrode assembly than the bottom.

[0112]As described above, the winding direction is reversed, in principle,
after each step is finished, the step including steps of: fabricating a
negative electrode; vapor-depositing lithium; cutting the electrode;
winding a positive electrode, a negative electrode and a separator; and
the like. As for the battery of the invention, it will suffice if the
electrode assembly is wound such that the head of the columnar particles
is finally positioned at the outer round side Do of the electrode
assembly than the bottom in the finished battery. If the electrode
assembly fabricated through the production process as described above has
such a winding direction, an extra step of winding again is not needed.
However, in the case where the head of the columnar particles is not
positioned at the outer round side Do of the electrode assembly than the
bottom by the foregoing production process only, a step of winding again
must be inserted in the production process once or an odd number of
times. By doing this, the winding direction of the electrode assembly can
be adjusted as appropriate.

[0113]It is preferable that a positive electrode lead and a negative
electrode lead are connected with the positive electrode and the negative
electrode, respectively, before an electrode assembly is formed. The
resultant electrode assembly is inserted into a predetermined battery
case (for example, a square or cylindrical battery can), and then the
positive electrode lead and the negative electrode lead are connected
with predetermined terminals (a battery can, a sealing plate, etc.).
Thereafter, a non-aqueous electrolyte is injected into the battery case.
Then the interior is evacuated to vacuum, thereby to allow the electrode
assembly to be impregnated with the non-aqueous electrolyte. Finally, the
battery case is sealed with a sealing plate, etc., whereby a battery is
finished.

[0114]The battery of the invention includes a lithium secondary battery of
various shapes, such as cylindrical, flat, and rectangular shapes. The
battery shape and sealing type is not particularly limited. An example of
the structure of a cylindrical lithium secondary battery will be
hereinafter described.

[0115]FIG. 15 is a longitudinal cross-sectional view of a cylindrical
lithium secondary battery according to the invention. A band-shaped
positive electrode 151 and a band-shaped negative electrode 152 are wound
with a band-shaped separator 153 interposed therebetween, of which width
is larger than the both electrodes, thereby to form an electrode assembly
154. To the positive electrode 151, a positive electrode lead 155 made of
aluminum etc. is connected, and one end of the positive electrode lead is
connected to a sealing plate 157 with an insulating packing 156 made of
polypropylene etc. provided on the periphery thereof. To the negative
electrode 152, a negative electrode lead (not shown) made of cupper etc.
is connected, and one end of the negative electrode lead is connected to
a battery can 158 housing the electrode assembly 154. On the top and
bottom of the electrode assembly 154, an upper insulating ring (not
shown) and a lower insulating ring 159 are disposed, respectively. The
electrode assembly 154 is impregnated with an electrolyte (not shown)
having lithium ion conductivity. The opening of the battery can 158 is
closed with the sealing plate 157.

[0116]At least one of the positive electrode 151 and the negative
electrode 152 (for example, the negative electrode 152) includes a
current collector and an active material layer carried on at least one
face of the current collector. The active material layer includes
columnar particles having a bottom and a head, the bottom of the columnar
particles is adhered to the current collector, and the head of the
columnar particles is positioned at the outer round side of the electrode
assembly 154 than the bottom. In other words, the growth direction of the
columnar particles goes from the inner round side toward the outer round
side of the electrode assembly 154. An angle formed between a direction
from the bottom toward the head of the columnar particles and a direction
normal to the current collector is, for example, 20° or more to
70° or less.

[0117]Preferably, a first active material layer is carried on one face of
the current collector, and a second active material layer is carried on
the other face; and each of the active material layers has the structure
as described above. In this case, an angle formed between a component
parallel to the current collector of a growth direction of the columnar
particles included in the first active material layer, and a component
parallel to the current collector of a growth direction of the columnar
particles included in the second active material layer is, for example,
80° or more and 90° or less.

[0118]For the electrolyte, for example, various solid electrolytes or
liquid non-aqueous electrolytes having lithium ion conductivity may be
used. Although the liquid non-aqueous electrolyte is not particular
limited, the one prepared by dissolving a lithium salt in a non-aqueous
solvent is preferably used. It is desirable that the concentration of
lithium salt in the liquid non-aqueous electrolyte is 0.5 mol/L or more
and 2 mol/L or less.

[0119]For the non-aqueous solvent, for example, cyclic carbonates such as
ethylene carbonate and propylene carbonate; chain carbonates such as
dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate are
preferably used. Generally, a mixture solvent of cyclic carbonates and
chain carbonates is used. To the non-aqueous solvent,
γ-butyrolactone, dimethoxyethane, and the like may be added.
However, no particular limitation is imposed on the composition of the
liquid non-aqueous electrolyte.

[0120]For the lithium salt, for example, lithium hexafluorophosphate,
lithium tetrafluoroborate, an imide-lithium salt, and the like may be
used. Among these, a liquid non-aqueous electrolyte mainly containing
lithium hexafluorophosphate makes the battery characteristics favorable
as compared with liquid non-aqueous electrolytes mainly containing other
lithium salts. It is preferable that small amounts of lithium
tetrafluoroborate and an imide-lithium salt are used in combination with
lithium hexafluorophosphate.

[0121]No particular limitation is imposed on the separator and an outer
case, and materials used for various types of batteries may be used as
desired. For the separator, for example, a microporous film made of a
polyolefin, and the like may be used.

[0122]The invention will be hereinafter described in detail with reference
to Examples. It should be noted that the invention is not limited to the
following Examples.

Example 1

(i) Fabrication of Negative Electrode

[0123]For the negative electrode current collector, a 35 μm thick
copper foil available from Furukawa Circuit Foil Co., Ltd. having a
roughened surface (Ra=1.8 μm) was used. The surface roughness Ra is
specified in Japanese Industrial Standards (JIS B 0601-1994). For the
active material source, a massive silicon simple substance with high
purity (5N) was used.

[0124]The production apparatus as shown in FIG. 12 was used to
continuously form a negative electrode active material layer on a
negative electrode current collector of continuous length in the
following procedures. The vacuum chamber 121 in the production apparatus
120 was evacuated to vacuum. Thereafter, oxygen was introduced to the
interior of the vacuum chamber 121. A mass flow controller was used to
introduce oxygen to the interior of the vacuum chamber 121 therethrough.
The flow rate of oxygen was adjusted so that the degree of vacuum during
the formation of an active material layer became approximately 0.03 Pa.

[0125]In the oxygen atmosphere as described above, silicon simple
substance serving as the active material source was evaporated. First, an
electron beam with an accelerating voltage of -10 kV was irradiated to
the massive silicon simple substance using a 270-degree deflection type
electron beam source available from JEOL Ltd, to dissolve silicon. Then
the emission current of the electron beam was gradually increased to
produce a vapor of silicon.

[0126]The position of the opening of the shielding plate 128 was adjusted
so that an incident direction of the vapor of silicon and a direction
normal to the current collector formed an angle of 50 to 70°. The
vapor of silicon passed through the opening, together with oxygen, was
incident on the surface of the negative electrode current collector which
was moving obliquely from the transfer roller 125b until it reached the
periphery of the can roller 126. Thereafter, the current collector
carrying an active material layer was wound on the winding roller 127.
The thickness of the active material layer was controlled to be 15 μm.

[0128]Observation of the active material layer indicated that the active
material layer included columnar particles slanting with respect to a
direction normal to the current collector. The active material layer was
cut in a direction parallel to the direction normal to the current
collector and parallel to the growth direction of the columnar particles,
and a cross section (cross section C) of the active material layer was
observed with an electron microscope. The result indicated that the angle
β formed between the direction from the bottom toward the head of
the columnar particles and the direction normal to the current collector
was approximately 40°.

[0129]The negative electrode current collector carrying the active
material layer was cut in a band shape (width: 15 mm, length: 340 mm)
having dimensions suitable for fabricating an electrode assembly, which
was used as a negative electrode. In this step, the negative electrode
was cut out so that the component parallel to the current collector of
the direction from the bottom toward the head of the columnar particles
was in parallel with the longitudinal direction of the negative
electrode. Around one end portion of the negative electrode in its
longitudinal direction (an end portion located at the bottom side of the
columnar particles, not at the head side), a negative electrode lead was
welded on the back face of the negative electrode current collector, the
face not carrying the active material layer.

(ii) Fabrication of Positive Electrode

[0130]100 parts by weight of lithium cobaltate (LiCoO2) having a mean
particle size of approximately 10 μm serving as a positive electrode
active material, 3 parts by weight of acetylene black serving as a
conductive agent, 8 parts by weight of a polyvinylidene fluoride powder
serving as a binder, and an appropriate amount of N-methyl-2-pyrrolidone
(NMP) was sufficiently mixed, to prepare a positive electrode material
mixture paste.

[0131]The resultant paste was applied onto one face of a positive
electrode current collector made of a 20 μm thick aluminum foil,
dried, and then rolled to form a positive electrode active material
layer. The thickness of the positive electrode active material layer was
approximately 75 μm. Thereafter, the positive electrode current
collector carrying the active material layer was cut in a band shape
(width: approximately 13 mm, length: approximately 330 mm) having
dimensions suitable for fabricating an electrode assembly, which was used
as a positive electrode. Around one end portion of the positive electrode
in its longitudinal direction, a positive electrode lead was welded on
the back face of the positive electrode current collector, the face not
carrying the active material layer.

(iii) Fabrication of Electrode Assembly

[0132]The positive electrode and the negative electrode were wound with
the positive electrode active material layer and the negative electrode
active material layer opposed to each other and a separator interposed
between these electrodes, whereby a cylindrical electrode assembly was
formed. In this step, in order to position the head of the columnar
particles in the negative electrode active material layer at the outer
round side of the electrode assembly than the bottom, in the negative
electrode, the end portion having the negative electrode lead was used as
the winding axis side. In the positive electrode, the end portion not
having the positive electrode lead was used as the winding axis side.
Here, for the separator, a 20 μm thick microporous film made of
polyethylene was used. The relation between the electrode assembly and
the slanting direction of the columnar particles in the negative
electrode active material layer is shown in the column of Example 1 in
FIG. 16.

(iv) Fabrication of Battery

[0133]The resultant electrode assembly was inserted into a case made of a
laminated sheet including an aluminum foil, and then a liquid non-aqueous
electrolyte was injected into the case. For the liquid non-aqueous
electrolyte, the one prepared by dissolving LiPF6 at a concentration
of 1 mol/L in a solvent mixture of ethylene carbonate and diethyl
carbonate in a volume ratio of 1:1 was used. The case was evacuated to
vacuum, to allow the electrode assembly to be impregnated with the liquid
non-aqueous electrolyte, and then the case was sealed.

Example 2

[0134]A battery was fabricated in the same manner as in Example 1 except
that the electrode assembly was wound in such a state as shown in the
column of Example 2 in FIG. 16.

Comparative Example 1

[0135]A battery was fabricated in the same manner as in Example 1 except
that the electrode assembly was wound in such a state as shown in the
column of Comparative Example 1 in FIG. 16.

Comparative Example 2

[0136]A battery was fabricated in the same manner as in Example 1 except
that the electrode assembly was wound in such a state as shown in the
column of Comparative Example 2 in FIG. 16.

[Evaluation]

(Charge-Discharge Test)

[0137]The batteries fabricated in Examples 1 and 2 and Comparative
Examples 1 and 2 were subjected eight cycles of charge-discharge with a
charge-discharge rate of 0.1 C (a current value required for charging or
discharging electricity in an amount equivalent to the nominal capacity
in 10 hours). Thereafter, 100 cycles of charge-discharge were performed
with a charge-discharge rate of 1 C (a current value required for
charging or discharging electricity in an amount equivalent to the
nominal capacity in one hour). Here, the charge-end voltage was 4.05 V,
and the discharge-end voltage was 2.0 V.

[0138]With respect to batteries of Example 1 and Comparative Example 1,
relations between the discharge capacity and the number of
charge-discharge cycles when the discharge capacity at the first cycle is
assumed to be 100% are shown in FIG. 17A. Further with respect to
batteries of Example 2 and Comparative Example 2, relations between the
discharge capacity and the number of charge-discharge cycles when the
discharge capacity at the first cycle is assumed to be 100% are shown in
FIG. 17B.

[0139]As shown in FIG. 17A and FIG. 17B, it was confirmed that the
batteries of Examples 1 and 2 have higher capacity retention rates,
compared with the batteries of Comparative Examples 1 and 2.

(Shape of Battery)

[0140]With respect to the batteries of Examples 1 and 2 and Comparative
Examples 1 and 2, the degree of deformation of the battery resulted from
the foregoing charge-discharge test was measured with X-ray CT scanning
to compared the results. The measurement was carried out before the
charge-discharge was started and after the charge-discharge was performed
100 cycles. The ratio of the major axis (maximum diameter) to the minor
axis (minimum diameter) of the transverse cross-section of the electrode
assembly was determined. The results are shown in Table 1.

[0141]As shown in Table 1, as for the electrode assemblies in Examples 1
and 2, the degree of deformation of the electrode assemblies was small.
It is considered therefore that the troubles due to deformation of the
battery or the electrode assembly can be prevented. For example, it is
considered that troubles such as a reduction in capacity due to a partial
deformation of the electrode assembly can be prevented.

[0142]In the foregoing Examples, although the description was made about
the case where the active material layer was formed on one face of the
current collector, also in the case where the active material layer is
formed on both faces of the current collector, a battery capable of
minimizing the deformation of the electrode assembly and excellent in
charge-discharge cycle characteristics can be obtained.

Example 3

[0143]A negative electrode active material layer was formed on both faces
of the negative electrode current collector in a manner in conformance
with Example 1. The resultant current collector with the active material
layers was used to fabricate a cylindrical battery as shown in FIG. 15.

(i) Fabrication of Negative Electrode

[0144]After a negative electrode active material layer was formed on one
face of the negative electrode current collector in the same manner as in
Example 1, the electrode roll was detached from the winding roller 127.
The detached electrode roll was reversed and placed on the feeding roller
123, so that a negative electrode active material layer could be
continuously formed also on the back face of the negative electrode
current collector.

[0145]Here, the production conditions such as the amount of oxygen were
changed so that the thickness of the negative electrode active material
layer became 17 μm per one face and the composition of the active
material determined by XRF analysis became SiO0.4.

[0146]Observation of the active material layer indicated that the active
material layer included columnar particles slanting with respect to a
direction normal to the current collector. The angle β formed
between the direction from the bottom toward the head of the columnar
particles and the direction normal to the current collector was
approximately 40° on both faces. The angle α formed between
a component parallel to the current collector of the direction from the
bottom toward the head of the columnar particles in one of the active
material layers and a component parallel to the current collector of the
direction from the bottom toward the head of columnar particles in the
other one of the active material layers was 0°.

[0147]The negative electrode current collector carrying the active
material layer was cut in a band shape (width: 60 mm, length: 700 mm)
having dimensions suitable for fabricating an electrode assembly, which
was used as a negative electrode. In this step, the negative electrode
was cut out so that the component parallel to the current collector of
the direction from the bottom toward the head of the columnar particles
became in parallel with the longitudinal direction of the negative
electrode. Around one end portion of the negative electrode in the
longitudinal direction (an end portion located at the bottom side of the
columnar particles, not at the head side), part of the active material
layer was scraped off, and a negative electrode lead was welded to the
negative electrode current collector.

(ii) Fabrication of Positive Electrode

[0148]The same positive electrode material mixture paste as used in
Example 1 was applied onto both faces of a positive electrode current
collector made of a 20 μm thick aluminum foil, dried, and then rolled
to form a positive electrode active material layer. The thickness of the
positive electrode active material layer was approximately 60 μm.
Thereafter, the positive electrode current collector carrying the active
material layers was cut in a band shape (width: approximately 58 mm,
length: approximately 690 mm) having dimensions suitable for fabricating
an electrode assembly, which was used as a positive electrode. Around one
end portion of the positive electrode in the longitudinal direction, part
of the active material layer was scraped off, and a positive electrode
lead was welded to the positive electrode current collector.

(iii) Fabrication of Electrode Assembly

[0149]The positive electrode and the negative electrode were wound in the
same manner as in Example 1, to form a cylindrical electrode assembly. In
this step, in order to position the head of the columnar particles in the
negative electrode active material layer at the outer round side of the
electrode assembly than the bottom, in the negative electrode, the end
portion having the negative electrode lead was used as the winding axis
side. In the positive electrode, the end portion not having the positive
electrode lead was used as the winding axis side. For the separator, a 20
μm thick microporous film made of polyethylene was used.

(iv) Fabrication of Battery

[0150]The resultant electrode assembly was inserted into a cylindrical
battery can. One end of the positive electrode lead was connected to a
sealing plate with a polypropylene packing provided on the periphery
thereof, and one end of the negative electrode lead was connected to the
battery can. On the top and bottom of the battery assembly, an upper
insulating ring and a lower insulating ring were disposed, respectively.
Thereafter, the same liquid non-aqueous electrolyte as used in Example 1
was injected into the battery can. The battery can was evacuated to
vacuum to allow the electrode assembly to be impregnated with the liquid
non-aqueous electrolyte, and then the battery can was sealed.

[0151]The charge-discharge test of the battery thus fabricated was
performed in the same manner as in Example 1. The proportion (capacity
retention rate) of a discharge capacity after 100 cycles of
charge-discharge to a discharge capacity at the first cycle was 90%. The
shape of the battery after the charge-discharge test was checked with
X-ray CT scanning. As a result, no great change was observed in the state
of the electrode assembly.

[0152]In the foregoing Examples, although the description was made about
the case of cylindrical batteries, also in the case of square batteries,
a battery capable of minimizing the deformation of the electrode assembly
and excellent in charge-discharge cycle characteristics can be obtained
on the basis of the same principle as in the case of cylindrical
batteries.

[0153]The invention is effective also in the case where the columnar
particles have a complicated shape (for example, a zigzag shape or a
helical shape).

[0154]FIG. 18 is a set of diagrams showing an embodiment of the invention
in the case where columnar particles have a zigzag shape. FIG. 18(a) is a
partially developed diagram viewed from the bottom of one side of a
cylindrical electrode assembly 181. The electrode assembly 181 includes a
band-shaped first electrode 182, a band-shaped second electrode 183, and
a band-shaped separator 184 disposed between these electrodes. FIG. 18(b)
is a magnified schematic diagram of an area encircled by the dashed line
X in FIG. 18(a), showing a cross-section of the first electrode 182. The
first electrode 182 has a current collector 185 and an active material
layer 186 carried on one face of the current collector. The active
material layer 186 includes columnar particles 188, in which the head of
the columnar particles 188 is positioned at an outer round side (Do) of
the electrode assembly 181 than the bottom. The foregoing electrode 182
can be used to fabricate an electrode assembly, with the same effect as
that of the invention. It should be noted that in FIG. 18, although only
the diagrams in which the columnar particles are formed on one face of
the current collector are shown, the columnar particles may be formed on
both faces.

[0155]FIG. 19 is a set of diagrams showing an embodiment of the invention
in the case where columnar particles have a helical shape. FIG. 19(a) is
a partially developed diagram viewed from the bottom of one side of a
cylindrical electrode assembly 191. The electrode assembly 191 includes a
band-shaped first electrode 192, a band-shaped second electrode 193, and
a band-shaped separator 194 disposed between these electrodes. FIG. 19(b)
is a magnified schematic diagram of an area encircled by the dashed line
Y in FIG. 19(a), showing a cross-section of the first electrode 192. The
first electrode has a current collector 195 and an active material layer
carried on one face of the current collector. The active material layer
196 includes columnar particles 198, in which the head of the columnar
particles 198 is positioned at an outer round side (Do) of the electrode
assembly 191 than the bottom. The foregoing electrode 192 can be used to
fabricate an electrode assembly, with the same effect as that of the
invention. It should be noted that in FIG. 19, although only the diagrams
in which the columnar particles are formed on one face of the current
collector are shown, the columnar particles may be formed on both faces.

INDUSTRIAL APPLICABILITY

[0156]The invention is effective in a battery including a high capacity
active material, particularly in a lithium secondary battery. According
to the invention, during the expansion of the active material, the
pressure to be applied to the separator and the electrode can be reduced.
This makes it easy to maintain the shapes of the active material
particles and secure the micropores of the separator. The battery of the
invention is applicable, for example, to the power sources for personal
digital assistants, mobile electronic equipment, compact home electrical
energy storage apparatus, motorcycles, electric cars and hybrid electric
cars, and the like. However, there is no particular limitation on the
application.